Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is connected directly or through suitable gearing to the vehicle's wheels or tracks. Acceleration and deceleration of the transmission are controlled by varying the displacement of the main pump from its neutral position. The present invention relates generally to the hydrostatic transmission and, more specifically, to the hydraulic pump/motor having integrated valves for providing a smoother operation during the acceleration phase of the transmission operation near its neutral position.
The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with the pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. A charge pump is added to the hydraulic circuit in order to charge the closed-circuit with hydraulic fluid through check valves, thus making up for possible lost fluid due to internal leakage. Other valves can be added to the closed-circuit. For example, high pressure relief valves can be used to protect the hydrostatic transmission from overloading during its operation, bypass valves can be used to allow oil to be routed from one side of the transmission to the other side without significant resistance, and hot-oil shuttle valves can be used to reduce the loop temperature by connecting the low pressure side of the closed loop to a drain, thus allowing replenishment with fresh, cooled replacement hydraulic fluid.
In hydrostatic applications, an over center variable displacement axial piston pump is used. The displacement of the pump is determined by the size and number of pistons, as well as the stroke length. A control handle enables the operator to control the direction and amount of flow from the pump. When an operator pushes the handle in one direction, the pump delivers flow for one direction of motor operation. When an operator pulls the handle in the opposite direction, the pump delivers flow for the opposite direction. To avoid a rough, jerky start of the motor, the prior art has utilized an orifice with a fixed diameter that is added to the closed-loop circuit to widen the width of the dead band of the hydrostatic transmission. The dead band of a hydrostatic transmission is the non-response range of the transmission near its neutral position where the motor will not turn over due to internal cross-port leakage across the bypass orifice. The orifice creates a bypass flow passage for the closed-loop, increases the dead band of the transmission, and allows the motor to start moving smoothly when the transmission is originally at neutral position. The size of the orifice is very important and the optimum diameter can be determined by carefully checking the change of stoking effects on the machine due to the change of orifice diameter. The orifice can also be integrated onto other hydraulic components, for example the aforementioned valves, within the closed-loop circuit.
Although the additional bypass orifice helps a machine obtain smooth operation near the neutral position of the hydrostatic transmission, there are disadvantages if the bypass orifice is fixed. A fixed bypass orifice allows a certain amount of flow routed from the high pressure side to the low pressure side of the closed-loop during all phases of the transmission's operations. This unwanted cross-port leakage not only reduces the overall efficiency of the hydrostatic transmission, but also generates substantial heat that increases the operating temperature of the closed loop. This can cause a safety issue for the machine and reduces its service life. An additional cooling device can be added, but this increases the cost and presents possible encumbrances when space is limited. It is desired that an orifice only performs its cross-port bypassing near the neutral position of the hydrostatic transmission, and then is disabled during the continuous operation of the motor.
Prior art, such as U.S. Pat. No. 3,740,950 to Polaski sets forth an example of a valve block design for use in a hydrostatic transmission application that consists of a cross-port bypass passage and two check valves interconnected by a spring. Flow though the valve is shut off when the spring between the two check valves is compressed. When one of the check valves is seated, flow through the bypass passages, as well as all flow through the valve block, is obstructed. This valve block design does not allow continued charging fluid to reach the low-pressure side without use of separate make-up check valves. Another prior art reference, U.S. Pat. No. 6,295,811 to Mangamo et al., also sets forth a design, which utilizes a bypass orifice in a valve for use in hydrostatic transmission applications. This design differs from the present invention in that the orifice can be disabled, but separate check valves are needed.